This invention relates to apparatus and methods for measuring the strength of the air/fuel mixture supplied to a combustion system which operates on liquid or gaseous hydrocarbon fuel (referred to herein as a combustion system of the type specified).
The invention is particularly although not exclusively applicable to the measurement of the mixture strength of internal combustion engines, e.g. automobile engines. For some years now great pressure has been exerted on the automobile industries in several countries to reduce the amounts of noxious gas emitted from the exhausts of road vehicles, and the scope of legislation is widening progressively to cover all sizes and types of internal combustion engine.
A wide range of instruments has been developed for the measurement of the various components in engine exhaust gases such as carbon monoxide, nitrogen oxides, unburnt hydrocarbons, carbon dioxide and oxygen. These have been used for research and development testing to reduce the exhaust gas emissions, for the official testing of new vehicle models for compliance with the legal requirements, and for routine testing of random vehicles as they leave the production line.
Partly to minimise the quantity of NOx in the exhaust gases, and partly to attain improved fuel consumption, there is a growing tendency to operate gasoline engines with lean mixture strengths, i.e. with mixtures in which an excess of oxygen over that for stoichiometric combustion is present. Thus there is a growing demand for instruments for the ready determination of the mixture strength supplied to an engine. Such instruments must operate from the exhaust gases of the engine, and it is convenient to base the mixture strength determination on the direct measurement of the gas proportions present in the exhaust gases.
Existing methods mostly work satisfactorily for research, production and inspection tests within the factory. There is, however, a growing need for smaller, self contained, and portable instruments. These are needed to extend research and development tests for vehicles operating under a whole range of conditions on the road, including the effect of altitude, using various mixture control devices. At the same time, the need for strict enforcement of the emission laws continues during the service life of a vehicle, which creates a demand for relatively small, lightweight, self-contained, robust and portable instruments which are relatively easy to operate. Such instruments could facilitate garage maintenance, periodical car health tests, and police surveillance.
There is thus a need for a light, portable, self-contained instrument based on the analysis of the exhaust gas in an operating vehicle, and one which can be used to give a direct readout of the pertaining equivalence ratio or, with the addition of a suitable chart recorder, a continuous trace of equivalence ratio with time.
The term equivalence ratio is used to mean the ratio of the actual air/fuel ratio of the mixture supplied to the engine for combustion, to the stoichiometric air/fuel ratio for the fuel being used. Numeric values greater than unity for this ratio indicate weak mixture strength, i.e. that an excess of oxygen is present in the mixture. Values less than one indicates a deficiency of oxygen, i.e. a rich mixture.